Latent Heat (VCE SSCE Chemistry): Revision Notes

Latent Heat

Introduction

Have you ever wondered why sweating helps cool you down on a hot day or during exercise? When you perspire, water is released onto your skin's surface. This water then absorbs a large amount of heat energy from your body before evaporating into the air as a gas. This process is an excellent example of latent heat in action.

Combined with water's high specific heat capacity, latent heat makes water one of nature's most effective coolants. Understanding latent heat is crucial for explaining many phenomena, from how our bodies regulate temperature to how oceans maintain stable temperatures for marine life.

What is latent heat?

When you heat a solid substance at constant pressure, its temperature rises steadily until it reaches its melting point. At this point, something interesting happens: even though you continue adding heat energy, the temperature stops rising and remains constant whilst the solid melts. The same phenomenon occurs when a liquid reaches its boiling point. The temperature stays constant during boiling until all the liquid has evaporated into gas.

Think about boiling water in a pot. No matter how vigorously the water boils, it stays at $100°\text{C}$ until every last drop has turned to steam. The graph below demonstrates this clearly.

The flat horizontal sections (plateaus) on this graph show where phase changes are occurring. During these plateaus, the temperature doesn't change even though energy is continuously being added. This energy isn't being "wasted" - it's being used to change the state of the substance rather than increase its temperature.

Types of latent heat

There are two types of latent heat, corresponding to the two main phase changes:

Latent heat of fusion

The latent heat of fusion is the energy required to change $1$ mole of a substance from solid to liquid at its melting point. The word "fusion" means melting or joining together. This energy is needed to overcome some (but not all) of the forces holding the solid structure together.

Latent heat of vaporisation

The latent heat of vaporisation is the energy required to change $1$ mole of a substance from liquid to gas at its boiling point. This process requires more energy than fusion because the particles must completely separate from each other to form a gas.

Latent heat values for water

Water's fusion and vaporisation values

Water has a latent heat of fusion of $6.0 \text{ kJ mol}^{-1}$. This means that $6.0 \text{ kJ}$ of energy is needed to melt $1$ mole of ice at $0°\text{C}$. This energy disrupts the regular ice lattice structure by breaking some of the hydrogen bonds between water molecules. However, many hydrogen bonds remain in the liquid state, which is why liquid water still has considerable structure.

Water has a latent heat of vaporisation of $40.7 \text{ kJ mol}^{-1}$. This substantially larger value reflects the fact that $40.7 \text{ kJ}$ of energy is needed to convert $1$ mole of liquid water to steam at $100°\text{C}$. This much greater energy requirement is because all the hydrogen bonds between water molecules must be completely broken to allow the molecules to separate and form a gas.

Comparison with other substances

The table below compares water's latent heat values with those of hydrogen and oxygen:

Substance	Latent heat of fusion ($\text{kJ mol}^{-1}$)	Latent heat of vaporisation ($\text{kJ mol}^{-1}$)
Water	$6.0$	$40.7$
Hydrogen	$0.06$	$0.45$
Oxygen	$0.22$	$3.4$

Calculating energy changes during phase transitions

When you know a substance's latent heat value, you can calculate how much energy is needed to change its state. The formula is:

$q = n \times L$

where:

• $q$ is the heat energy in kilojoules ($\text{kJ}$)
• $n$ is the amount of substance in moles ($\text{mol}$)
• $L$ is the latent heat value (either fusion or vaporisation) in $\text{kJ mol}^{-1}$

The significance of water's high latent heat

Body temperature regulation

Water's high latent heat of vaporisation makes perspiration an extremely effective cooling mechanism. When sweat evaporates from your skin, each mole of water removes $40.7 \text{ kJ}$ of heat energy from your body. This large energy absorption causes significant cooling, helping to prevent overheating during exercise or in hot environments.

Water storage and conservation

Australia stores much of its fresh water supply in open reservoirs. Although some water is lost through evaporation, these losses would be far greater if water had a lower latent heat of vaporisation. The high energy requirement for evaporation means that sunlight doesn't cause rapid water loss from storage facilities.

Ocean temperature regulation

Ocean temperatures vary across the Earth, being warmer near the equator and cooler near the poles. However, the temperature in any particular ocean region remains relatively stable over time. This stability is crucial for marine organisms such as coral and fish, which are sensitive to temperature fluctuations.

The global ocean currents shown above illustrate temperature variations, with red colours indicating warm water and green colours showing cooler regions. The average ocean surface temperature is about $17°\text{C}$, ranging from around $-2.2°\text{C}$ in the Arctic Ocean (salt dissolved in seawater lowers the freezing point) to between $19°\text{C}$ and $30°\text{C}$ in the Indian Ocean.

Two properties of water work together to maintain stable ocean temperatures:

1. High specific heat capacity means that even intense sunlight doesn't cause large temperature increases in ocean water
2. High latent heat of vaporisation means that high levels of sunlight don't result in excessive evaporation

These properties create a stable aquatic environment that supports diverse marine ecosystems.